Acid rain is a problem throughout the world. Acid rain affects the environment by reducing air quality, rendering lakes acid and killing vegetation, particularly trees. It has been the subject of international dispute. Canada and the United States have argued over the production of acid rain. European countries are other antagonists.
In the main, acid rain stems from sulphur dioxide produced in smoke stacks. The sulphur dioxide typically originates from the sulphur containing fuel, for example coal. The sulphur dioxide is oxidized in the atmosphere to sulphur trioxide and the sulphur trioxide is dissolved to form sulphuric acid. The rain is thus made acidic. The oxides of nitrogen are also a factor in producing acid in the atmosphere. Millions of tons of oxides of nitrogen are fed to the atmosphere each year.
With the passage of international clean air acts, such as issued in the United States in 1990, the reduction of acid emissions has become a priority. Planners for electrical utilities in particular are developing strategies for reducing emissions of sulphur dioxide and nitrogen oxides in the production of electrical and thermal power. The majority of fossil fuel used in power production contains sulphur which produces sulphur dioxide and hydrogen sulphide during combustion.
In an effort to improve economics for electric power production from coal and production of concrete, as well as eliminate metal containing solid waste discharges to landfills, there is an increasing desire to recycle the ash combustion products of fossil fuel combustion, especially that related to char or coal combustion.
Naik et al (ref. 14) describes the beneficial effects of low carbon content coal ash on the performance of concrete. High calcium containing coal ash was successfully used to replace up to 50% of Portland cement in concretes with a variety of enhanced properties including improved durability such as cracking resistance.
Malhotra and Mehta (ref. 11) indicated that "Portland cement is the most energy-intensive component of a concrete mixture, whereas pozzolanic and cementitious by-products from thermal power production and metallurgical operations require little or no expenditure of energy. Therefore, as a cement substitute, typically from 20% to 60% cement replacement by mass, the use of such by-products in the cement and concrete industry can result in substantial energy savings. Concrete mixtures containing pozzolanic and cementitious materials exhibit superior durability to thermal cracking and aggressive chemicals. This explains the increasing worldwide trend toward utilization of pozzolanic and cementitious materials either in the form of blended portland cements or as direct additions to portland cement concrete during the mixing operation." These authors classify ash products as follows:
Pozzolans--"A pozzolan is a siliceous or siliceous and aluminous material, which itself possesses little or no cementitious property but which will in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperature to form compounds possessing cementing properties." PA1 Cementitious--"there are some finely divided and non-crystalline or poorly crystalline materials similar to pozzolans but containing sufficient calcium to form compounds which possess cementing properties after interaction with water. These materials are classified as cementitious." PA1 a) Fouling of exterior solid surfaces by calcium sulphite or calcium sulphate; PA1 b) Absorption of heat due to evolution of carbon dioxide (from calcium carbonate) or steam (from calcium hydroxide) resulting in lower furnace temperatures, reduced rates of fossil fuel burning, reduction of furnace power output per unit of fuel input; PA1 c) Desulphurization is restricted to the "post flame combustion region" which is associated with the "sintering" or "collapse" of calcium oxide crystals at temperatures of about 1200.degree. C. resulting in a loss of their porosity. Loss of lime porosity is clearly identified by the Simons reference (see ref. 17) as highly detrimental to sulphur dioxide adsorption; PA1 d) The desulphurization is restricted to the formation of calcium sulphate or calcium sulphite; PA1 e) The lime/sulphur reaction which occurs in the gas-solid state, in the post combustion zone is slow, resulting in inadequate sulphur dioxide removal and inadequate residence times for sulphur dioxide removal. The lime sintering problem therefore requires precise narrow temperature region injection of the reagent e.g. &lt;1200.degree. C.; and PA1 f) No byproduct ash recycling in a value-added form is possible. In fact the ash is contaminated with a calcium sulphate byproduct contaminated with unreacted internal lime which results in an undesirable landfill problem due to residue alkalinity. PA1 a) High CaO content; PA1 b) Low temperature; PA1 c) A fluid slag this is promoted by CaF.sub.2 additions and avoiding excessively high slag acidities or operation below the melting point of the slag; PA1 d) CaF.sub.2 additions--these not only increase fluidity, but also increase the fundamental rate of the desulphurization reaction; and PA1 e) Stirring in the bath due to gas bubbles. PA1 a) Although high iron levels in a coal have often been associated with ash deposition and slagging (fouling), they are not definitive with respect to potential for such behaviour; PA1 b) Whether iron mineral is predominantly in the form of pyrite FeS.sub.2 or siderite FeCO.sub.3 is "included" or "excluded" nature, is closely associated with included silicate and aluminosilicate minerals, and the combustion conditions to which it is subject are important factors when considering such minerals potential for ash deposition and slagging; PA1 c) Coals containing pyrite mineral have the potential to produce ash deposition and slagging at lower temperatures than do coals containing siderite material; PA1 d) Under reducing conditions coals containing iron minerals pyrite and siderite have the potential to produce ash deposition and slagging problems at lower temperatures than for oxidizing conditions; and PA1 a) CaO--SiO.sub.2 --Al.sub.2 O.sub.3 --FeO slags react with char to produce CO and with reaction rate increasing with increasing FeO content; PA1 b) CaO, CaCO.sub.3 or CaSO.sub.4 catalytically enhance char combustion rates by 2700, 190 and 290 times respectively if they are in intimate contact with char. Molten CaO and other Ca containing species including CaF.sub.2, CaSO.sub.4 etc. are clearly catalysts for oxidation of coal carbon to CO via ionized calcium carbide formation CaC.sub.2. Achieving intimate contact between the molten Ca species is stressed again and again as the key to maximizing the benefit of this desirable catalytic effect. Well dispersed CaO, especially in the presence of CO has been found to be efficient in both sulphur capture and NOx reduction e.g. NO and N.sub.2 0 reduction. Optimum desulphurization in oxide melts such as those containing CaO are enhanced in the presence of CaF.sub.2 and stirring of the melts due to gas evolution (e.g. CO gas evolution). CaF.sub.2 enhances the reactivity of CaO melts by reducing their viscosity and increasing their reactivity especially in the presence of FeO and/or SiO.sub.2 or their melts; PA1 c) CaO or CaO/CaF.sub.2 containing melts have the ability to eliminate or reduce fouling problems due to sticky FeO--Al.sub.2 O.sub.3 --SiO.sub.2 containing melts derived from pyrite FeS.sub.2 or siderite FeCO.sub.3 containing coals in pulverized coal combustors due to their ability to depolymerize silicates thereby making them less viscous (non-sticky); PA1 d) CaF.sub.2 solubilizes CaO/C decomposition products i.e. CaC.sub.2 thereby indirectly increasing catalytic C oxidation via CaO; and PA1 e) Current low NOx combustor technology is incompatible with the production of valuable low carbon pozzolanic and/or cementitious ash for purposes of concrete production due to undesirable unburned carbon levels in the ash. PA1 a) enhanced coal combustion, especially under Low NOx combustor operating conditions; PA1 b) enhanced acid emission reduction due to desulphurization; PA1 c) maximization of the pozzolanic or cementitious value of fossil fuel ash, especially coal ash; PA1 d) enhanced ability to use a wider variety of coals or chars for production of pozzolanic or cementitious ash by-products, especially those currently unsuitable for use due to unburned carbon contents; PA1 e) minimization or elimination of combustor fouling due to combustor operation under Low NOx operating conditions especially in cases where iron rich coal or char containing siderite FeCO.sub.3 or pyrite FeS.sub.2 is present; and PA1 f) potential recycling of low-value or land filled high carbon ash in a novel, more cost effective process in a manner which enriches its calcium content thereby dramatically increasing its cementitious or pozzolanic value.
Ramme in U.S. Pat. No. 5,992,336 (ref. 15) indicated that "a principal reason for the lack of commercial value for coal ash is the presence of unburned carbon in the ash (page 1, lines 18-20). He describes "reburning" of coal ash as the only cost effective alternative to reducing carbon content of coal ash.
Frady et al (ref. 7) also describe a process for upgrading the pozzolanic value of ash using a fluidized bed ash reburning process to reduce its carbon content. They acknowledged a desire to promote the use of coal ash in concrete production. They indicated that without their ash reburning technology "ash carbon content was marginal at best and non-saleable to the concrete market at worst". In addition they "recognized that changes in combustion conditions designed to meet low NOx regulations would lead to a further diminishment in fly ash quality. As quality was already marginal at several stations, further diminishment would essentially shut this fly ash out of the local concrete market, which was strong and growing."
Gas desulphurization systems are known. The majority rely on simple basic compounds such as calcium carbonate, calcium oxide or calcium hydroxide, to react with the acidic sulphur containing species to produce non-volatile products such as calcium sulphite and calcium sulphate.
Conventional alkaline adsorbents such as calcium carbonate and calcium hydroxide undergo thermal decomposition to calcium oxide at high temperature, which results in the chemical reaction of calcium oxide with sulphur dioxide. However, the adsorbents suffer from a number of problems:
This technique for desulphurization has not been accepted to any degree by the coal-fired power industry.
The prior art has described laboratory experiments with respect to catalytic destruction of NOx. For instance, Illan-Gomez et al. (ref. 10) investigated the catalytic destruction of NO on carbon surfaces in the presence of Cao. They indicated that well dispersed CaO formed upon pyrolysis of lignite coals was found to be efficient in both in-situ sulphur capture and NOx reduction. They described the effectiveness of calcium loaded carbon in NOx reduction in the presence of molecular oxygen O.sub.2. The catalytic role of calcium was found to be analogous to the role it has in carbon gasification, that of increasing the concentration of carbon-oxygen complexes on the carbon surface.
Aarna and Suuberg (ref. 1) demonstrated the enhancement of NO reduction on coal char by CO. They described reports concerning the catalysis of the following reaction by various types of surfaces including calcined limestone (CaO) and CaO used in sulphur retention: EQU NO+CO=1/2N.sub.2 +CO.sub.2
The steel industry has described techniques for desulphurization in molten alkaline CaO environments.
For instance, Ward (ref. 20) summarized conditions for optimum desulphurization via oxide melts:
The prior art have described laboratory experiments involving impregnation of devolatilized chars including coal chars, with CaO precursors such as calcium containing salt solutions, such as calcium acetate, to increase char combustion rates. The steel industry has illustrated the impact of molten CaO containing mixtures on carbon containing char oxidation rates of interest to that industry.
For instance, Sarma et al. (ref. 16) showed that CaO--SiO.sub.2 --Al.sub.2 O.sub.3 --FeO slags react with char at 1400 to 1450.degree. C. to generate CO. Reaction rate increased with increasing FeO content of slag. A gas film formed between the slag and the surface. CaO/SiO.sub.2 weight ratio was unity. The diffusion of Fe.sup.2+ and O.sup.2- ions from the bulk of the slag to the slag-gas interface is at least one of the rate limiting steps for the overall reduction reaction. EQU C+FeO=Fe+CO EQU FeO+CO=Fe+CO.sub.2 &lt;1535 Celsius EQU CO.sub.2 +C=2CO EQU Fe+C=FeC
Gopalakrishnan et al. (ref. 9) showed the catalytic oxidation of char by CaO, CaCO.sub.3 and CaSO.sub.4 at 1200.degree. C. The results indicated significant catalytic effects of up to 2700 times for CaO, 160 times for CaCO.sub.3 and 290 times for CaSO.sub.4. Oxidation rate increased with increasing CaO loading in char pores.
Song et al. (ref. 18) described the thermodynamic behaviour of carbon in CaO--SiO.sub.2 slags. They implied a carbon reaction mechanism involving reaction of carbon with oxygen ions supplied from CaO in the slag. The solubility of carbide in CaO.SiO.sub.2 slag increased with addition of CaF.sub.2. It was speculated that the presence of fluoride ions increased CaO basicity (electronegativity) by depolymerizing silicate ion networks via replacement of polymer bridging oxygen ions with non-polymer bridging fluoride ions.
The dissolution mechanism for carbon was expressed as follows: EQU nC+mO.sup.2- =C.sub.n.sup.2m- +m/2O.sub.2
where C.sub.n.sup.2m- represents carbide or in the form of complex ion of carbonate e.g. EQU C+O.sup.2- =C.sup.2- +1/2O.sub.2 EQU 2C+O.sup.2- =C.sub.2.sup.2- +1/2O.sub.2 EQU C+1/2O.sub.2 =CO
Overall: EQU 3C+CaO=CaC.sub.2 +CO EQU CaC.sub.2 +3/2O.sub.2 =CaO+2CO
Molten CaO has therefore been demonstrated as a catalyst for the oxidation of carbon to CO via formation of an ionized calcium carbide intermediate. This latter reaction is based on the solubility of carbon increasing with increasing slag basicity. Carbon solubility was found to increase with increasing temperature.
Gopalakrishnan and Bartholemew (ref. 9) determined the effect of CaO with respect to carbon structure and coal rank on char oxidation rates. They indicated that catalysis of char oxidation by CaO is an accepted fact and that char oxidation in the presence of CaO increased with decreasing char "skeletal density". They indicated that CaO catalyzes gasification by O.sub.2, CO.sub.2 and H.sub.2 O of low-rank coal chars and that the importance of well-dispersed CaO and intimate carbon-CaO contact is well established. They investigated quantitatively the effect of calcium oxide catalysis on the reactivity of Dietz sub-bituminous coal char prepared under high-temperature conditions representative of pulverized coal combustion.
Zhang et al. (ref. 23) demonstrated the effect of iron oxides such as Fe.sub.2 O.sub.3 and FeO in the catalytic gasification of sub-bituminous coal chars in the presence of carbon dioxide as follows: EQU FeO+C=Fe+CO EQU Fe.sub.2 O.sub.3 +C=2FeO+CO EQU CO.sub.2 +Fe=FeO+CO EQU CO.sub.2 +2FeO=Fe.sub.2 O.sub.3 +CO
Overall: EQU C+CO.sub.2 =2CO
The prior art has described the beneficial effect of fluoride in CaO containing melts of interest to the steel industry. For instance, Zaitsev et al. (ref. 21) describe the thermodynamic properties and phase equilibria for CaF.sub.2 --SiO.sub.2 --Al.sub.2 O.sub.3 --CaO melts. This reference clearly describes the polymerization/depolymerization behaviour of silica as silicates in silica containing melts e.g. SiO.sub.2 forms Si.sub.3 O.sub.9.sup.6-, Si.sub.6 O.sub.18.sup.-12 and so on. The Zaitsev reference indicates that the following reaction is possible in CaF.sub.2 --CaO--Al.sub.2 O.sub.3 melts: EQU CaO.sub.melt +3C.sub.solid =CaC.sub.2 +CO
Zaitsev et al. (ref. 22) further indicate species present in CaF.sub.2 --CaO--Al.sub.2 O.sub.3 --SiO.sub.2 melts where the following abbreviations are used C=CaO, A=Al.sub.2 O.sub.3, S=SiO.sub.2. They indicated that the CaF.sub.2 --CaO-Al.sub.2 O.sub.3 --SiO.sub.2 melt consisted of monomer, associative and polymer species. Associative species include:
CA, C.sub.2 S, CS, AS, C.sub.2 AS, CAS and CAS.sub.2
Polymer species include SiO.sub.2 networks connected with AS (e.g. AS.sub.y where y.gtoreq.2) or CAS (e.g. CAS.sub.z where y.gtoreq.2)).
Ueda and Meda (ref. 19) described the behaviour of CaF.sub.2 in the presence of silicates. They indicated that CaF.sub.2 decreases the melting point of a mixture of calcium oxide and silicates and thereby increases its reactivity. This reference indicated that a small amount of Al.sub.2 O.sub.3 in a CaO--CaF.sub.2 mixture improved the ability of CaF.sub.2 --CaO to dissolve SiO.sub.2.
Edmunds and Taylor (ref. 3) described the kinetics of the reaction between CaO--Al.sub.2 O.sub.3 --CaF.sub.2 melts and carbon. These authors showed that CaO--Al.sub.2 O.sub.3 --SiO.sub.2 or CaO--Al.sub.2 O.sub.3 melts react with graphitic carbon via the following reaction: EQU CaO+3C=CaC.sub.2 +CO
This reference allows shows that CaC.sub.2 is soluble in molten CaF.sub.2 (e.g. 0.22 moles CaC.sub.2 with 0.78 moles CaF.sub.2 at 1500.degree. C.)
The prior art has studied combustor fouling properties associated with the inorganic iron, sulphur and ash components of coals. For instance, McLennan et al. (ref. 12) have indicated that North American coals contain iron predominantly in the form of pyrite FeS.sub.2. Asian coals have iron mainly in the form of siderite FeCO.sub.3. McLennan et al. described the decomposition of iron containing species in coal including pyrite FeS.sub.2 and siderite FeCO.sub.3. They suggested that included FeS.sub.2 particles embedded in char would be exposed to a reducing environment even though the external char surfaces could be exposed to oxidizing conditions. Therefore, oxidation of "occluded" or "included" FeS in char generated by thermal decomposition of "occluded" FeS.sub.2 would not proceed to any great extent until the completion of char combustion. This delay in the oxidation of "included" FeS.sub.2 or FeS accounted for the significant number of Fe--O--S ash particles of high FeS content identified for oxidizing combustion geometry. Ash particles derived experimentally from high pyrite containing coals were found to have high FeS content for this reason even under oxidizing conditions. They concluded that "exposed" or "excluded" FeS.sub.2 decomposes to FeS, then oxidizes from the surface inward to produce a molten FeO--FeS phase at 1080.degree. C., which will oxidize to Fe.sub.3 O.sub.4 and Fe.sub.2 O.sub.3 under oxidizing conditions, but remain as FeO--FeS under reducing conditions. "Included FeS.sub.2 may behave as for excluded pyrite if there is no contact with aluminosilicates, though oxidation will be delayed by char combustion. Included pyrite that contacts aluminosilicate materials will form two phase FeS/Fe-glass ash particles, with incorporation of iron into the glass as the FeS phase is oxidized. This delay in glass formation is expected to be accentuated by reducing conditions." In a subsequent reference, McLennan et al. (ref. 13) studied pulverized combustor fouling effects due to sticky iron containing deposits derived from iron containing coals. They concluded the following:
e) For air staged combustion (see above discussion on Low NOx burners), where reducing conditions exist in the lower regions of the furnace, the potential for deposition and slagging due to molten ash particles will be greater than that for conventional combustion under oxidizing conditions. Based on the melting temperatures of the ash formed, the increase in ash deposition and slagging will be greatest for pyrite containing coals, moderate for coals with a high degree of mineral association, and slight for siderite containing coals.
The prior art has studied factors impacting "stickiness" or "non-stickiness" related to the viscosities of melts associated with iron silicate and iron aluminosilicate chemistry in the presence and absence of alkali such as CaO. For instance, Waseda and Toguri (ref. 24) have described the structure and properties of oxide melts, especially those relating to viscosity. "General features are that the viscosity of oxide melts decrease with increasing temperature and the ratio of network modifier component to network former one, reflecting the situation of silicate anions which consist of a flow unit. Viscosity of oxide melts is influenced primarily by the content of network former which give large complex anions. Silicate is a typical network former that has SiO.sub.4.sup.4- as its fundamental structural unit. Viscosity is intimately related to the size and shape of the silicate anions. The fundamental structural unit can undergo a series of polymerization reactions as the silica content of the melt increases. The so-called basic oxides which act as network modifiers lower the viscosity of melts by breaking the bridge in the Si--O network structure. This makes the anionic structural units of silicates smaller, resulting in a decrease in the viscosity of silicate melts." These authors described the effect of fluoride substitution on the viscosity of CaO--SiO.sub.2 melts. They stated that fluorides lower the viscosity about twice as much as CaO. They also described the viscosity of FeO--SiO.sub.2 melts. As expected, the viscosity of FeO--SiO.sub.2 melts rises as the SiO.sub.2 /FeO ratio increases. For FeO--SiO.sub.2 mixtures, decreases in viscosity were observed for all melts upon the addition of CaO. The decrease is more prominent for high silica melts, which suggests that CaO modifies the Si--O bonds rather than the FeO bonds.
In summary the prior art has identified the following factors relevant to fossil fuel combustion, especially that related to coal combustion:
The prior art however, especially related to coal combustion technology, has failed to incorporate knowledge derived in the steel industry to its requirements. Furthermore, its attempts to use the desirable effects of CaO have been restricted to impregnation of devolatilized coals in laboratory experiments with calcium containing aqueous solutions. Clearly this method of impregnation is unsuitable for anything but devolatilized char containing combusted coal ash. The prior art has failed to reveal how its problems related to ash fouling, desulphurization, NOx control and ash recycling can be solved simultaneously using simple and cost effective techniques which eliminate the current apparent requirement for ash reburning.
Accordingly, it is an object of the current invention to provide an improved method for the achievement of one or more of the following objectives: